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copied!<p>A unit test would test <code>SomeClass</code> at most. Therefore you would typically mock classWithEvent. Using some kind of injection for classWithEvent in ctor is fine. Just as Thomas Weller said wiring is field assignment.</p> <p>Option 2 is actually bad IMHO. As if you omit a call to <code>ActivateEventHandling</code> you end up with a improperly initialized class and need to transport knowledge of the requirement to call <code>ActivateEventHandling</code> in comments or somehow else, which make the class harder to use and probably results in a class-usage that was not even tested by you, as you have called <code>ActivateEventHandling</code> and tested it but an uninformed user omitting the activation didn't, and you have certainly not tested your class when <code>ActivateEventHandling</code> was not called, right? :)</p> <p><strong>Edit: There may be alternative approaches here which are worth mentioning it</strong></p> <p>Depending on the paradigm it may be wise to avoid wiring events in the class at all. I need to relativize my comment on Stephen Byrne's answer.</p> <p>Wiring can be regarded as context knowledge. The <a href="http://en.wikipedia.org/wiki/Single_responsibility_principle" rel="nofollow">single responsibility principle</a> says a class should do only one task. Furthermore a class can be used more versatile if it does not have a dependency to something else. A very loosely coupled system would provide many classes witch have events and handlers and do not know other classes.</p> <p>The environment is then responsible for wiring all the classes together to connect events properly with handlers. The environment would create the context in which the classes interact with each-other in a meaningful way. A class in this case does therefore not know to whom it will be bound and it actually does not care. If it requires a value, it asks for it, whom it asks should be unknown to it. In that case there wouldn't even be an interface injected into the ctor to avoid a dependency. This concept is similar to neurons in a brain as they also emit messages to the environment and expect answer not knowing neighbouring neurons.</p> <p>However I regard a dependency to an interface, if it is injected by some means of a dependency injection container just another paradigm and not less wrong.</p> <p>The non trivial task of the environment to wire up all classes on start may lead to runtime errors (which are mitigated by a very good test coverage of functional and integration tests, which may be a hard task for large projects) and it gets very annoying if you need to wire dozens of classes and probably hundreds of events on startup manually. While I agree that wiring in an environment and not in the class itself can be nice, it is not practical for large scale code.</p> <p>Ralf Westphal (one of the founders of the <a href="http://www.clean-code-developer.de/" rel="nofollow" title="CCD in German;">clean code developer initiative (sorry german only)</a>) has written a software that performs the wiring automatically in a concept called "event based components" (not necessarily coined by himself). It uses naming conventions and signature matching with reflection to bind events and handlers together.</p>

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